I've not so much to say and I've already written my thoughts.
About PO4 measuring, I use Hanna PO4 low range photometer with Elos or Rowa or Tunze reagents. I put 5 drops and 1 spoon in hanna vial (half spoon for Elos). It is very important to gently mix the solution without trapping bubbles. The vial must be perfect without any scratches. I mix for 3 minutes, make immediate read and then 3 minutes delay test. The second test is always slightly higher then the first one. You can try 10 consecutive tests and results will be always the same.
About nutrients, I can say what it's wrong on the basis of my experience and, if You want, I can say What I think it should be right, according to all information I got from others experience, threads, TOTM, etc, in the 16 years I move in reef tanks world.
I can say it's wrong to have zero nutrients (low/zero organic and zero inorganic - mean 0 NO3 and zero PO4), lot of light (duration and intensity) and a big skimmer.
In the last 2 years I've used to give 12h full light (12x54w t5), small amount of food, few fishes and a killer skimmer (BK supermarine 200 in 420 net liters volume).
Some corals have had good colors, some ddidn't; some grew, some didn't. Every time I saw things worsen I tried to increase organics and reduced light. Growth reduced, corals darkened. So I increased light and stop feeding. Tank seemed to start back, but was only a temporary situation. In the last 2 years I lost various corals very, very slowly. In the pictures You can see a big green milly that in the course of 2 years has slowly recessed till die. Other corals showed tissue thinning in the surfaces exposed to light wile reduced slowly growth. Many frags have never grown, in 2 whole years.
Here and there I found that most of thriving and old tanks use much less light than me, specially in duration, give much more food and have NO3 and PO4 detectable. Mines weren't detectable for 2 whole years.
So I decided to make quite a drastic change. I increased food. I reduced light to 6h full light and 12h only x2 blue+ tubes. I changed to a smaller skimmer.
PO4 are now 0,01 - 0,02. NO3 0,2 - 0,5 (salifert), after about 1 month.
Some of the starving corals recovered and are just starting to grow. Many had STN. New frags are instead doing fine. It's like if "old" corals were so unhealty they can't recover any more.
Tank is now a mess, but I trust it will recover and improve with much patience.
My opinion is that light "consumes" organic, directly or undirectly, and drive to them to wrong creatures. So we must use it with caution. Feeding is important, but without strong light, it causes increase of inorganics. An healthy tank with organics and inorganics houses healthy corals, maybe not light coloured, but healthy and with good growth. A tank with too few organics houses starving corals with inconstant health and grow and can't last too long.
I just make a list of people using low light (comparing to my standard) I've read recently and with proven results (random order):
dvanacker
jroover
Lunar
Cunareefer
reefbum
Thomas Phol uses to keep his fixtures at 40cm above his tanks, with 8h light. When we personally asked him what would happen using zeovit with fixture 10cm above tank, He answered we were fool and that that was a good way to kill corals.
One of the best italian tank I personally saw more than one time measures 250x100x95cm, ...95 cm tall. It's owner uses 4 8x54w ati fixtures, 30cm above tank. His rockwork it's only 40cm high and corals grow even in semishadow on bottom. He runs biopellets.
Here some pictures.
http://www.acquariofilia.biz/showthread.php?t=412398&highlight=paolo+broggi
My sensation is that organics presence is a must-to-have to keep healthy. Colors can improved lowering inorganics with carbon sources, like BP or liquid carbon. Like if low light pushes the coral to a more Heterotrophic behaviour and carbon sources simply transform inorganic to another form of organics that isn't available to zoox, but it is to corals.
I stress
my sensation, not certainty.
But all good tanks have in common low light intensity/duration and much food, with detectable inorganics, not zero both NO3 and PO4.
